In power transmission systems and then especially in High Voltage Direct Current (HVDC) power transmission systems, the voltages used in power transmissions are getting higher and higher. Today 800 kV is used in many situations. However, nowadays even higher levels are contemplated, such as 1000 kV or even 1200 kV.
The use of such high levels lead to various types of problems. Some such problems are discussed by Davidson and de Preville in “The Future of High Power Electronics in Transmission and Distribution Power Systems”, 13th European Conference on Power Electronics and Applications, EPE 2009, 8-10 Sep. 2009 Page:1-14.
One specific type of problem is the insulation. This has for instance been studied by U Åström, B Westman, V Lescale and G Asplund in “Power transmission with HVDC at voltages above 600 kV”, Inaugural IEEE PES 2005 Conference and Expositions in Africa, Durban, South Africa, 11-15 Jul. 2005, page 44-50.
In such systems it is also known to provide the valves of converter stations as a number of interconnected outdoor converter valve elements. Such elements are for instance described in EP 0754367 and U.S. Pat. No. 5,371,651.
As mentioned above one problem that arises when designing a system that is to operate at very high voltages, is the insulation. Extraordinary measures may need to be made in order to ensure insulation that can handle the required voltage levels, which makes the equipment bulkier, associated with extensive development costs and will lead to more complicated designs of bushings and general insulation.
There is therefore a need for improving the way insulation is provided for power transmission system equipment that can be used at very high voltages, such as voltages at or above 1000 kV.